Sheave assembly for offshore drilling rigs



. sHEAvE ASSEMBLY FOR OFF-SHORE DRILLING'BIGS i:

' Original Filed Ju1y 5, I967 5 Sheets-Sheet 1 INVENTOR EDWARD E HORTON SHEAVE ASSEMBLY FOR OFFSHORE DRILLING mes Originai F'il ed July s, 1967 May 12,1970 'HQRTQN s shets sheet z M;y-12 ,1970 E. raj- ORTON 7 3, 11,

. v SHEAVE ASSEMBLY FOROFFSHOREDRILLING RIGS' Original Filed July 5, 1967 5 Sheets-Sheet a POSITION SIGINAL SIGNAL MIXER I E. E. HORTON 3,511,476

I SHEAVE ASSEMBLY'FOR OFFSHORE DRILLING R168 7 Original Filed July 5,1967

1 May 12 7 I .5 shees-sheet 4 PRioR ART I May1'2,1970

" or gihalfile July 5. 1967 E! E. HORTON 3,511,476 SHEAVE ASSEMBLY FOR OFFSHORE DRILLING RIGS 5 Sheets- Sheet 5 United States Patent 3,511,476 SHEAVE ASSEMBLY FOR OFFSHORE DRILLING RIGS Edward E. Horton, Palos Verdes, Calif., assignor to Ocean Science and Engineering, Inc., Washington, DC, a corporation of Delaware Original application July 5, 1967, Ser. No. 651,194. Divided and this application June 19, 1968, Ser.

Int. Cl. B66d N48 US. Cl. 254139 Claims ABSTRACT OF THE DISCLOSURE A sheave assembly for use in offshore drilling operation for maintaining the vertical location of a drill pipe stable with respect to the ocean floor even though its floating support platform is subject to wave action. The heave assembly comprises an assembly received for vertical slidable movement and supported by suitable framework on said platform. A pulley is carried by the assembly and is flanked on opposite sides of a first sheave and a second sheave respectively. Each sheave is spaced laterally from said assembly and is supported by said framework at an elevation corresponding to the midpoint of said assembly movement range. A cable is aflixed at one end to the platform and extends over the first sheave, around said pulley and over said second sheave. By such an arrangement the rotation of the pulley upon vertical movement of the assembly is kept at a minimum. The elfective length of the cable being varied by increasing or decreasing the angle through which the cable must travel from said sheaves to or from said pulley. Further, the first and second sheaves serve to minimize the load placed on the pulley.

This application is a division of copending application Ser. No. 651,194, filed July 5, 1967.

This invention relates to apparatus for stabilizing the tension and compression forces acting on a drill pipe sus-' pended from a floating vessel during an offshore drilling operation. The art of conducting drilling operations from ships and stations at points over the bed of the ocean has become increasingly active in recent years. When the drilling is commenced at great depths, it is impractical to stabilize the drilling platform with members extending to the ocean floor. Such drilling stations or platforms therefor are subject to the wave actions of the sea. Although certain measures are available to reduce the effect of this wave action through ship design and buoy systems, the principal eifect of this wave action is always present. Such heaving action causes thedrill pipe and drill bit to be alternately pulled upwardly from the drilling area as the platform rises and dashes the bit against the bottom of the boring as the platform drops. This invention provides apparatus for mounting the drill pipe to the platform in a manner to maintain stability with respect to the ocean floor by substantially isolating the wave action from the drill pipe.

A principal object of the invention is to provide a crown block support means for the drill pipe which means includes apparatus causing the crown block to remain relatively fixed with respect to the ocean floor as its mounting platform heaves.

Another object of the invention is to provide a crown block supporting means which utilizes pressurized air as a primary support means and hydraulic fluid as both a secondary support means and a power means.

A further object of the invention is to provide a crown 3,511,476 Patented May 12, 1970 supporting means block which quickly compensates for wave action through a hydraulic power system controlled by an accelerometer detector which anticipates this wave action.

A still further object of the invention is to provide a sheave support system for a cable supported crown block having minimum sheave rotation when corrective motlons are necessary.

Another object of the invention is to provide a crown block which is adjustable to proportionately compensate for only a segment of platform heave when the wave action is so severe that it cannot be fully compensated because of physical limitations in the supporting derrick.

These and other objects of the invention will become more apparent to those skilled in the art by reference to the following detailed description when viewed in light of the accompanying drawings, wherein:

FIG. 1 is a front elevational view of the invention;

FIG. 2 is a side elevational view of the invention;

FIG. 3 is a perspective view of the crown block assembly;

FIG. 4 is a front elevational view of the crown block assembly;

FIG. 5 is a schematic of the crown block air and hydraulic system;

FIG. 6 is a schematic of the crown block control system;

FIGS. 7a and 7b are a schematic of a prior art sheave support system in dilferent positions;

FIGS. 7c and 7d are a schematic of the instant sheave support system in two different positions, and

FIGS. 8a, b and c are diagrammatic elevational views of the ship in three difierent positions.

Referring now to the drawings, wherein like numerals indicate like parts, the numeral 10 indicates a ship or other floatable platform which has a derrick 12 rigidly mounted on its deck. The derrick is of a type which is well known in the art and has a water table platform 14 at its upper end and a working board 16 therebelow. The derrick is used to support a drill string 18 by means of a travel block 20 and a crown block assembly 22 over and through which pass the cables or lines 23. In order to prevent twisting of the lines 23, outwardly extending arms 24 and 24 are provided to slidably engage a pair of guide rails 26 and 26'.

Assembly 22 is located within a derrick superstructure 28 and is supported -by the water table platform 14. The superstructure comprises a framework 30 having vertical legs 32 which are rigidly braced by diagonal beams 34. The crown block assembly 22 includes a pair of opposed horizontal bars 36 which rotatably support a plurality of crown block sheaves 38. The assembly 22 is supported within the framework by a plurality of vertically disposed air pistons 41. A plurality of rollers 40 are mounted on the assembly and engage the inner corners of the legs 32 to thus permit relative movement between the assembly 22 and the framework 30.

The pistons 41 have their rods 42 secured to cross bars 36 and their cylindrical portions 46 to the table 14 via pins 47. The air cylinders are communicated to pressurized air storage tanks or flasks 50 located on the working board 16 by air pressure lines 48. The pressurized air provides a resilient means of supporting the major portion of the crown block load. -In addition to its weight, the load includes the suspended drill string 18.

The crown block assembly is further supported by a plurality of double acting hydraulic jacks 54. First rods 52 of the jack are slidably received through the upper end of cylinders 56 and are secured to bar 36 by pin connections 47. The base of cylinders 56 are connected to water table 14 also by pin connectors although the lower ends of cylinders 56 have balance rods 52, which are not connectedto any part of the-apparatus. The hydraulic jacks can move the assembly to any selected position intermediate the vertical length of framework 30.

The double acting hydraulic pistons 54 are used not only as a secondary support means for taking up load difference not taken up by the air pistons 44 but primarily provide the means for moving the crown block in compensating synchronization with the vertical motion of the ship. It is advantageous to use a double-acting, doubleended jack with the expansion chambers interconnected since the displaced volume of fluid from the contrasting side of the cylinder will equal the increase in volume expanding chamber. As best seen in FIG. 5, opposite ends of the cylinder 56 are communicated by hydraulic lines 58, 58' to a variable displacement pump 60.

In the event of a sudden change in crown block load due to a sudden break in the drill string or fracture in either the air or hydraulic systems, a dash pot 62 (FIG. 4) is provided on the Work table with its dash pot rod 64 secured to the bar 36. Although not shown, the interior of the dash pot cylinder is grooved to allow adequate velocity of the crown block at the middle of its vertical movement and controlled deceleration at the end of it. A spring biased relief valve 66 (FIG. 5) will operate upon a sudden load change to pass fluid to the low pressure side of the hydraulic system and thus simultaneously transfer the load to the dash pots 62. During normal operation, the dash pots are passive and have a negligible effect.

As briefly mentioned above, the line 23 passes about the crown sheaves 38. As best seen in FIGS. 3 and 4, an idler sheave 68 is rotatably supported rigid web plates 70. The idler sheave 68 supports the dead line portion 72 of the line 23. An idler sheave 74 is similarly disposed on the opposite end of the work table 14 between the web plates 76 and supports the fast line portion 78 of the line 23. A schematic of the instant sheave support system is shown in FIGS. 70 and 7d and discloses that the dead line 72 is secured to the derrick floor while the fast line 78 passes around a fast sheave 80 on the derrick floor and thence to a draw works drum (not shown) which maintains a constant preselected tension therein. The purpose of the idler sheaves is to minimize rotation of the traveling block sheaves and the crown block sheaves to thus reduce line and sheave wear. In FIGS. 7a and 7b, a sheave support system is shown without the idler sheaves. It can be seen when the ship moves down from the FIG. 7a to the FIG. 7b position, the compensating vertical movement of the crown block causes a relatively large partial rotation of the crown and travel sheaves. The sheave support system (FIGS. 70 and 7d) utilizes the idler sheaves disposed on the water table at an elevation substantially equal to the midpoint of the crown sheaves vertical movement limits. The portion of the dead line and the fast line between the idler sheave and crown sheave are indicated by the numerals 72 and 78 respectively. When the ship moves from the FIG. 70 to the F16. 7d position the idler sheaves move downwardly with the ship while the crown sheaves are compensated upwardly. The line portions 72' and 78' are thus moved outof their substantially horizontal positions to provide awcomponent of the increased dead line and fast line lengths. In other words, in FIG. 7a, the increase in distance, between the derrick floor and the crown sheaves is supplied partly by a swinging movement of the dead line andfast line portions 72 and 78' and partially by longitudinal lengthening of the dead and fast lines 72, 78 .whereas in FIG. 7b, the increase in distance is wholly supplied by a longitudinal lengthening of the dead and fast lines 72, 78.

From the foregoing discussion, it is apparent that the reducedrotation of the crown and travel sheaves gained by the use of idler sheaves will result in minimum line and sheave wear. It is also seen that since the dead line and fast line come off of the crown sheaves on the horizontal, their loads are not supported by the air and hydraulic pistons of the crown block assembly.

For effective operation the speed of crown block compensation must be synchronized with the speed of the ships heave 14. For these purposes a control system as shown in FIG. 6 is provided; An accelerometer 82 on the crown block 22' senses vertical motion and transmits an electric signal to a servo valve 84. The signal will be positive or negative depending on the direction of the vertical movement. The servo controls the output of a hydraulic accumulator 86 and directs it through either of the lines 90 or 90 to the valve 88 to determine flow of the pump 60 through one of the two lines 58, 58'.

Drift of the crown block from its mid position is prevented by the use of a conventional electrical signal device wherein a displacement signal is originated at 94. The said displacement signal is proportional to the distance the crown block is from the mid-point and acts as a small override to keep the crown block centered.

During heavy seas, the ship heave motion may exceed the allowable travel of the crown block and it is therefore desirable to compensate for only a portion of the heave. To accomplish this, a second accelerometer 96 is provided on the water table. The signal from the second accelerometer has its sign reverse-d at 98. The water table signal is thus subtracted from the crown block signal at a signal mixer 100 and the ditference will then go to they servo valve 84. The amplification of the water table signal takes place at 102 and is adjustable by the operator. If the absolute values of the two signals are equal, no compensation takes place. If, for example, the water table signal is set at 80 percent of the crown block signal, the crown block is compensated only to 20 percent.

The operating principles are best seen in FIGS. 8a, 8b and 8c which depict compensation of the crown block location during different stages of ship heave. The movements discussed will relate to movements relative to the sea floor unless otherwise specified.

During a mean position as shown in FIG. 8a, the crown block 22 with its hook load is supported near its mid-point in the framework 30 by the air pistons 44 and the hydraulic jacks 54. As long as the ship is stationary, the crown block will be stationary also. As the ship heaves upwardly to the FIG. 8b position, the inertial force of the load supported by the air pistons 44 will resist upward movement and cause air compression in the cylinders 46. The natural resistance to upward movement, however, is lessened by the increasing pressure of the air, and the crown block will begin an upward travel. The accelerometer 82 (FIG. 6) will sense this travel and cause hydraulic fluid to flow into upper chambers of cylinders 56 to maintain the crown block stationary. Similarly, when the ship moves downwardly to the FIG. 80 position, any tendency of the crown block to move downwardly is sensed by the accelerometer 84 and hydraulic fiuid is pumped to the lower chambers of the cylinders 56. v

What has been set forth above is intended primarily as exemplary to enable those skilled in the art in the practice of the invention and it should therefore be understood that, within the scope of the appended claims,

responding to the mid-point of said assembly movement range,

a second sheave laterally spaced on the opposite side of said assembly from said first sheave and sup ported by said framework at an elevation corresponding to the mid-point of said assembly movement range,

a cable having a deadline portion aifixed to said platform and extending from said platform to said first sheave, an intermediate portion extending from said first sheave around said pulley to said second sheave, and a fast line portion extending from said second sheave to said platform,

power means mounted on said framework for vertically sliding said assembly, and

tension means connected to said fast line portion for maintaining constant tension therein.

2. The hoisting mechanism of claim 1 wherein said assembly comprises opposed spaced bars slidably mounted in said framework and shaft means connecting said bars and rotatably supporting said pulley.

3. The hoisting mechanism of claim 2 wherein said power means is a hydraulically operated reciprocating motor, one end of said motor being connected to said framework and the other end to one of said bars.

4. The hoisting mechanism of claim 1 and including resilient support means between said framework and said assembly normally supporting said assembly intermediate the vertical length of said framework.

5. The moisting mechanism of claim 4 wherein said resilient support means is a pneumatic piston and cylinder arrrangent.

References Cited UNITED STATES PATENTS 1,928,958 10/1933 Young 254190 2,587,638 3/1952 Meier 254173 2,601,611 6/1952 Hilborn 254190 2,609,181 9/1952 Jaeschke 254173 HARVEY C. HORNSBY, Primary Examiner US. 01. X.R. 

